High-technology ceramics are subjects of growing interest in view of their excellent mechanical properties under stress, outstanding electrical and optical properties, and exceptional resistance to high temperatures and corrosive environments. They are useful in automobile engines, burner nozzles and heat exchangers. In view of their electrical properties, they are useful in capacitors, piezoelectric devices, thermistors, solar cells and integrated circuit substrates. Ceramics can also be used in lasers, cutting tools and bearings.
The present invention relates to bonding of ceramics to metals or other ceramics to provide a joint of high strength. Presently, there are several known methods for joining ceramics to metals or ceramics to ceramics. One of the most popular known techniques is the molybdenum-manganese process. In this process, a finely divided mixture of molybdenum and manganese powder is applied to the surface of a ceramic member, and the member is then fired in a reducing atmosphere at about 1400.degree. C. (2250.degree. F.) to sinter the metal powder to the ceramic surface. To this metallized surface, an easily solderable metal layer, such as nickel, may be applied by conventional techniques such as electroplating. A copper-silver eutectic material is then frequently utilized to braze the ceramic to a metal component.
In another popular technique generally referred to as the "active metal process", metallizing of the ceramic surface is achieved by coating the surface with titanium or zirconium hydride powder, placing a suitable brazing material over the layer of powder, and then firing in a vacuum at about 900.degree. C. (1650.degree. F.) to dissociate the hydride and form a bond between the ceramic and metal. This coated ceramic surface may then be bonded to a metal component. Unfortunately, the metallizing temperature range of 1600.degree. to 2600.degree. F. required for these processes is excessive for either alumina or zirconia because it provokes grain structure changes and also weakens the mechanical properties of the ceramic member. Nevertheless, these technologies are considered to be well established and widely used on a commercial basis.
Various other techniques have been used to attempt the bonding of a metal to a ceramic member wherein a variety of metals and/or alloys are initially bonded to the surface of the ceramic member to provide a metallized surface preparatory to the bonding of the metallized surface to the metal substrate. The present invention provides an improved joining process without the necessity of the extra steps to metallize the ceramic component.